In the art of blast furnace operation it is well known to reduce the consumption of coke by injecting pulverized coal into the hot blast in the blast furnace tuyeres. Such an injection system typically comprises a conveying hopper located at a first location, generally in proximity of a pulverized coal preparation plant, a fluidizing device for fluidizing the pulverized coal at the outlet of the conveying hopper and a pneumatic conveying line connecting the fluidizing device to a distribution device located at a second location, generally in proximity of the blast furnace. In the distribution device, the pneumatic flow is split between several injection lines, which are connected to injection lances arranged in the blast furnace tuyeres for injecting the pulverized in to the hot blast. It will be noted that the distance between the first location (also called upstream location hereinafter) and the second location (also called downstream location hereinafter) generally equals several hundred meters and often exceeds 1 km.
In order to warrant constant process conditions in the blast furnace, the quantities of pulverized coal injected into the blast furnace must be precisely adjustable and should not be subjected to major fluctuations. Different methods for mass flow rate control in such injection systems have been developed so far. According to a first method, the mass flow rate is controlled by adjusting the gas pressure in the conveying hopper either responsive to the output signal of a differential weighing system equipping the hopper or responsive to the output signal of a mass flow rate sensor mounted directly in the pneumatic conveying line. According to a second method, the mass flow rate is controlled by adjusting the flow rate of the fluidizing gas injected into the fluidizing device of the conveying hopper or the flow rate of dilution gas injected into the pneumatic conveying line either responsive to the output signal of a differential weighing system equipping the conveying hopper or responsive to the output signal of a mass flow rate sensor mounted directly in the pneumatic conveying line. According to a third method, the mass flow rate is controlled by throttling the pneumatic flow by means of flow control valve. According to a first embodiment of this third method, a main flow control valve is mounted in the conveying line at the conveying hopper location, i.e. in the start section of the pneumatic conveying line, and controlled responsive to the output signal of a differential weighing system equipping the conveying hopper or responsive to the output signal of a mass flow rate sensor mounted in the conveying line at the conveying hopper location. According to a second embodiment of this third method, an injection flow control valve is mounted in each of the injection lines at the distributor location and controlled responsive to the output signal of an injection mass flow rate sensor mounted in the respective injection line.
U.S. Pat. No. 5,123,632 discloses a pneumatic injection system for injecting pulverized coal into a blast furnace. The system comprises two conveying hoppers located at an upstream location. The total flow rate of the pulverized coal to be injected into the furnace is regulated in a metering apparatus at the outlet of each conveying hopper. This metering apparatus is connected by a main pneumatic conveying line to a static distribution device, which is located at a downstream location near the blast furnace and which is e.g. of the type described in U.S. Pat. No. 4,702,182. In this distributor, the primary pneumatic current is split into secondary currents which are conveyed through injection lines to the blast furnace tuyeres. Each injection pipe comprises a closing valve and at least one flow rate control tuyere. It is proposed to maintain in each injection line a constant pressure downstream of the first flow rate control tuyere, either by a pressure controlled injection of a compensating gas or by a pressure controlled valve in the injection line downstream of the first flow rate control tuyere.
U.S. Pat. No. 5,285,735 discloses a system for controlling the injection quantity of pulverized coal from a pressurized feed tank into a pneumatic conveying line, which conveys the pulverized coal to a blast furnace. This document suggests to install a powder flow meter in the conveying line near the pressurized feed tank to measure the flow rate of the pulverized coal flowing into the pneumatic conveying line. The output signal of this powder flow meter is used by a so called flow indicating controller to control the opening of a powder valve installed between the feed tank and the pneumatic conveying line. Alternatively, the flow indicating controller may use the output signal from a weighing system equipping the pressurized feed tank for controlling the opening of the powder valve.
Recent tests carried out by the Applicant of the present application have shown that—despite state of the art mass flow rate control—the mass flow rate in the conveying line and the injection lines is surprisingly subjected to important fluctuations. Applicant has found out that these fluctuations in mass flow rate are the more important the longer the pneumatic conveying line is.